ONE of Josie Gay's fondest memories growing up in Watts was riding the ``red car'' trolley to school. Now, nearly a half century later, she is glad her grandchildren will be able to experience something similar - riding the ``blue line'' train that will begin operation in her neighborhood tomorrow. Light-rail service is returning to Los Angeles in what may become the country's premier test of whether people can be induced to ride instead of drive.

For the first time in 29 years, trolleys will begin running along a 22-mile stretch between Long Beach and Los Angeles on Saturday. It is the first leg of what is planned to be a 150-mile web of rails across the region, the second largest in the country.

The success or failure of the ``blue line'' will help determine the level of political support for the rest of the rail network here, which includes a costly and controversial subway under construction downtown. It will also be closely watched across the nation.

At least 10 cities are building or considering light-rail systems. If Los Angeles, the cradle of car culture, can attract riders and keep them safe on a train that traverses tough gang territory, it will become a powerful argument to those pushing trains as a form of mass transit.

Failure will give rail critics - of which there are plenty - new conviction in their belief that there are cheaper antidotes to traffic congestion in America.

``A lot will ride on the success or failure of the Long Beach line,'' says Kenneth Orski, president of Urban Mobility Corporation, a Washington, D.C., consulting firm.

The $877 million line will open tomorrow to huzzahs from politicians and bands and bunting at community festivals along the route. Residents will be allowed to ride the electric street cars free over the weekend. Monday, it will become a paying railroad, with fares being a flat $1.10 rate anywhere a passenger gets on.

The spur runs from downtown Long Beach, past foundries and furniture stores, past the window-barred bungalows of Compton and Watts, and on to the edge of downtown Los Angeles.

It marks the beginning of what planners hope will be a fundamental shift in the way southern Californians get about. As part of the planned 150-mile Metro Rail network, officials envision:

An automated light-rail train, already under construction, that will run 20 miles from Norwalk to El Segundo down the middle of the Century Freeway. It is slated to open in 1994.

A subway, the $3.9 billion centerpiece of the system, that will stretch from downtown Los Angeles north to the San Fernando Valley. The first four miles of the subway are scheduled to open in 1993.

Three other lines, probably all light rail, will run through the San Fernando Valley, out to Pasadena, and along the coast.

So far, officials have funding for the Long Beach and Century Freeway legs. They are being underwritten by local sales taxes. Money has also been earmarked for the first two segments of the subway, about half coming from Uncle Sam and the rest from state, local, and private sources.

The impetus for this rush to rails is congestion on the freeway. Studies show the average speed on area highways to be 43 m.p.h. If current trends hold, that is projected to drop to 22 m.p.h. by 2010.

Transportation planners and many politicians consider the train, in addition to car-pooling, working at home, and other steps, essential to easing both gridlock and air pollution.

``There isn't a major metropolis in the world that doesn't have a rail component to their transportation network,'' says Neil Peterson, executive director of the Los Angeles County Transportation Commission (LACTC), builder of the blue line.

Others see things differently. Critics contend this quintessential sprawling city doesn't have the population density to support a rail system. They argue that public money would be better spent on expanding bus service and more aggressively promoting efficient use of highways.

``All things considered, people will walk away from rail,'' says Peter Gordon, a professor of urban planning at the University of Southern California. ``The convenience of the automobile wins everytime.''

Many cities have been turning to trolleys. Modern systems now exist in San Diego, Sacramento, San Jose, and Portland, Ore. Others building or looking at them include Dallas, Salt Lake City, St. Louis, Kansas City, Minneapolis, and Milwaukee.

Officials hope the blue line will draw 12,000 riders a day by the end of the year and 54,000 by the end of the century. The white steel-skinned cars are expected to make the 22-mile trip, which currently has 17 stops, in just under an hour.

``We need to make this train more attractive than the automobile,'' says Norm Jester, LACTC vice president of engineering. ``The only way we can do that is reduce the commute time.''

Safety is a concern. An estimated 15,000 gang members live within one mile of either side of the line. To deal with this, 118 sheriff's deputies are being assigned to the corridor - probably more per mile than any transit system in the country. Police will also use surveillance cameras to monitor activity at each train station. They believe crime and graffiti can be kept in check.

Josie Gay isn't so sure.

``Kids are not like they used to be,'' she says, standing in the heart of what was once called ``charcoal alley,'' the area of Watts that burned during the 1965 riots but which has been rebuilt. ``People do anything now. They may even shoot at it [the train].''

The last trolleys stopped running here in 1961, victims, in part, of Californians' love affair with the car. Will their modern incarnation fare better?